This invention relates to the field of nozzles for turbo-machinery. More particularly, this invention relates to the field of convergent-divergent nozzle structure for the creation and expansion of supersonic flow of a compressible fluid for turbine motive fluid.
Converging-diverging nozzles are needed to create and expand a supersonic stream (a pressure ratio exceeding approximately 1.85) to transform a high energy stream into a high velocity jet with good efficiency and minimal shock, flow separation or jet deflection.
Conventional prior art nozzles for convergent-divergent supersonic expansion are of two general types. One type has convergent-divergent profiles machined into opposed side surfaces or side walls of adjacent nozzles so as to define, in one dimension, convergent-divergent passages between these profiled side surfaces. The top and bottom surfaces of such passages are parallel to each other to define a constant height for the convergent-divergent passage, or the height may vary linearly between nozzle inlet and exit. The other type of conventional nozzle is in the form of rounded nozzle passages, as used in drilled and reamed nozzle blocks. The passages machined into these nozzle blocks may be circular in cross-section and thus two-dimensionally convergent-divergent. In any case, the contours of the nozzle passages, including convergence, divergence and throat location, are not defined by the inner and outer circumferential walls of the nozzle passage; rather, they are defined either by the contouring between the side walls in the nozzle or by the uniform annular variations of the drilled and reamed passages.
Both of these prior art nozzle configurations, i.e., those in which the converging, throat and diverging sections, are defined either by spacing between contoured adjacent side walls of nozzle elements or by the circumferential changes in the drilled and reamed passages, are characterized by several problems and disadvantages of long standing in the art. Cost is a particular problem in that precise machining must be employed to achieve the desired side wall contours where the passages are being defined by the contours between adjacent side walls; and, similarly, expensive drilling and reaming is involved for the drilled and reamed devices. In addition, the entire unit can be rendered useless by a mistake in the machining of other forming operations, and tolerances are particularly critical.
Another particular problem is that these prior art devices present distinct limitations in regard to flow capacity. In the round nozzles, for example, increased capacity is achieved by enlarging the contoured circular passages, thus also increasing the overall size of the unit. Assuming that the nozzles are arrayed in a circumferential array for communication with a turbine wheel, an attempt to increase capacity invariably results in enlargement of the overall size of the turbo-machinery beyond merely a diameter change, a result which is often very undesirable or which may result in excessive tip speeds. Similarly, if the nozzle arrays are associated with a radial turbine rather than an axial turbine, the increased size of the nozzle array required for increased flow capacity results in a disproportioning in size between the nozzle unit and the turbine wheel and may lead to inefficient delivery of motive fluid to the turbine wheel.